EPDM rubber, an elastic terpolymer of ethylene, an alpha-olefin such as propylene, and a diene such as ethylidene norbornene, has the molecular structure not including an unsaturated bond in the main chain and displays superiority to general conjugated diene rubbers in regards to weather resistance, chemical resistance, heat resistance, and so forth. Due to the characteristics, the elastic terpolymer such as EPDM rubber has been widely used for industrial materials, such as materials for all sorts of automobile parts, electric wires, hoses for construction or other use purposes, gaskets, belts, bumpers, a blend with plastics, etc.
The elastic terpolymer like EPDM rubber has been prepared mostly by copolymerizing three different monomers in the presence of a catalyst including a vanadium compound, such as a vanadium-based Ziegler-Natta catalyst. But the truth is that the preparation of the elastic terpolymer using copolymerization in the presence of a vanadium-based catalyst according to the conventional method not only leads to great deterioration in the catalytic activity of the catalyst for copolymerization but also makes it difficult to obtain an elastic terpolymer with sufficiently high molecular weight and good mechanical strengths.
Furthermore, the elastic terpolymer like EPDM rubber is required to have good elasticity and flexibility, but the conventional elastic terpolymers are, in many cases, poor in both elasticity and flexibility. The reason is presumably that the characteristic of the vanadium-based catalyst being suitable for low-temperature polymerization makes it difficult to control the reaction temperature. Another disadvantage of using the vanadium-based catalyst lies in that it is hard to control the up-take of the comonomers such as propylene, diene, etc.
To overcome the problems, there has been a sustained demand for developing an elastic terpolymer having good mechanical properties and elasticity (flexibility) at the same time and a method for preparing the same more effectively.